Conventionally, an electron beam exposure apparatus has long been used to produce masks that are the templates for semiconductor devices such as DRAMs and MPUs (microprocessing units). In the last several years, as advances in the resolutions that these electron beam exposure apparatuses are capable of achieving have led to ever-denser semiconductor production processes, such electron beam exposure apparatuses have been applied to exposure devices used in the lithographic part of the production process. Currently, a so-called direct draw-type electron beam exposure apparatus has been proposed as an apparatus capable of being adapted to the design rules of 4-gigabit DRAMs and more, in which electron beams discharged from an electron gun are concentrated and directed by a deflecting system, an electromagnetic lens or the like at the point of concentration onto a semiconductor substrate so as to draw directly on the substrate.
However, there are several problems with attempting to adapt such an apparatus to the semiconductor device mass production process, of which the most important is the drawing speed, with a high throughput of from several tens to several hundreds of times that of a so-called mask drawing unit required. As one means of solving this problem there is a so-called multi-beam-type electron beam exposure apparatus, in which the electron beam discharged from the electron gun is divided into a plurality of beams, for example 1,000, and arranged in the form of a matrix, and the beams used simultaneously to draw on the substrate specimen. A multi-beam electron beam exposure apparatus's ability to draw patterns simultaneously across a wide area using a plurality of electron beams can achieve dramatic improvements in through-put.
This type of apparatus, that is, an electron gun that draws directly on the substrate using a plurality of electron beams arrayed over a broad area, requires a certain electron beam intensity in order to draw a pattern directly with the divided electron beams. Moreover, because a single electron beam is divided into a plurality of beams over a wide area, the angular current density distribution of the beam must be flat and therefore the emittance must also be large. Ordinarily the intensity and the emittance are conservative levels whose values are determined by the electron gun that serves as the light source of the apparatus.
The basic structure of the electron gun employed in the conventional electron beam exposure apparatus involves a cathode in which the tip is shaped into a projection or sharpened to a point in order to increase the intensity, a Wehnelt for concentrating the electrons emitted from the cathode, to which is applied an electric potential lower than the voltage applied to the cathode, and an anode having a ground electrode. This tripolar type of electron gun is simple and easy to operate, and is widely used. However, because this type of electron gun is geared to increasing the intensity of the beam, and therefore it is very difficult to satisfy the required flatness of angular current density distribution over a wide area as described above.
In order to solve the foregoing problem, for example, Japanese Laid-Open Patent Publication (Kokai) No. 2000-285840 discloses a tripolar structure in which the cathode (the electron discharge surface of which is shaped into a hemisphere), the bias electrode and the anode are aligned on the optical axis, with a distance from a center of an aperture in the bias electrode to the tip of the electron discharge surface of the cathode being approximately equal to or slightly greater than a radius of the aperture of the bias electrode, and as a result achieving the desired flatness of angular current density distribution while maintaining relatively high intensity. FIG. 6 shows one example comparing the angular current density distributions of the electron gun described above and an electron gun having the typical tripolar configuration. In the diagram, 101 indicates the angular current density distribution of the typical tripolar configuration and 102 indicates the angular current density distribution of an electron gun employing a hemispherical cathode. As can be seen from the diagram, the electron gun employing the hemispherical cathode has a larger flat portion than that of the electron gun employing the typical tripolar configuration. By optically processing and then spatially dividing this flat portion, the electron gun employing the hemispherical cathode can provide the type of multiple high-intensity electron beams described above.
It should be noted that similar technologies are disclosed in other publications, for example, Japanese Laid-Open Patent Publication (Kokai) No. 9-129166, Japanese Laid-Open Patent Publication (Kokai) No. 9-180663 and Japanese Laid-Open Patent Publication (Kokai) No. 9-260237.
When laying particular emphasis on accuracy in actual lithography, it is necessary simultaneously to draw while controlling any drift in the characteristics of the apparatuses and to increase the resolving power of the parameters. The electron gun described above uses only the flat portion of the angular current density distribution, with the remaining portions being cut off at an appropriate location by an aperture. Such shielding of the current generates heat, and is cooled by a variety of methods to control temperature changes. At the same time, the drawing accuracy demanded of apparatuses of this generation does not permit fluctuations in the performance of the electro-optical system due to slight changes in temperature. For this reason, it is preferable that the absolute amount of the cut-off current be as small as possible.
In addition, as patterns have become finer, so too, the importance of the accuracy of various fine adjustments during drawing, such as proximity effect correction, auxiliary exposure for form correction, etc., has increased. For example, it is necessary to fine-tune the exposure energy and draw the pattern and auxiliary pattern so as to further improve form accuracy. In order to achieve this objective, it is necessary to increase the control resolving power of the exposure energy amount of the drawing apparatus as much as possible. From this standpoint, it is preferable to be able to constantly change the intensity of the electron gun to suit the process and other such drawing conditions.
However, in the electron gun of the tripolar structure described above, intensity adjustment is accomplished by fine adjustment of either the anode voltage or the bias voltage. Of these two methods, that of finely adjusting the anode voltage is difficult to adapt to lithographic apparatuses because the energy of the electrons ultimately obtained changes. In addition, with the bias voltage adjustment method as well, typically, as the intensity changes the angular current density distribution also changes. FIG. 7 shows one example of the relation between the bias voltage V1 and the angular current density distribution, in a tripolar electron gun having a rounded cathode. As can be understood from the graph in FIG. 7, the intensity changes as the bias voltage V1 changes, and at the same time the angular current density distribution also changes, changing the flatness. If the electron beam is split under these conditions, the strength of the individual beams obtained from the split can be uneven, thus degrading the drawing characteristics of the apparatus.